Add deadline to prevent further changes from staff.

app/messages/__init__.py

 from .message_builders import (
+    deadline_expired_for_staff,
     deadline_expired_for_students,
     login_manager_message,
     login_unsuccessful_error,
@@ -17,3 +18,4 @@ PROJECT_NOT_FOUND = project_not_found()
 SHORTLIST_NOT_FOUND = shortlist_not_found()
 
 DEADLINE_EXPIRED_FOR_STUDENTS = deadline_expired_for_students()
+DEADLINE_EXPIRED_FOR_STAFF = deadline_expired_for_staff()